K02 (server-build-plan.md) — Go ↔ Soroban integration spike

Status: complete, 2026-08-06. Confirms a Go service can build, sign, and submit Stellar transactions against the K01 contract end to end — using the Stellar Go SDK directly (github.com/stellar/go), no soroban-cli, no shelling out to another process. This is what services/core-go will actually do in Phase 1.

Deploys a fresh instance of K01's already-compiled wasm (../k01-soroban-escrow/target/wasm32v1-none/release/k01_soroban_escrow.wasm, reused as-is, not rebuilt) and drives it through the same lifecycle K01 validated, entirely from Go: upload wasm → create contract → initialize → fund → approve → NEG-1 → release → NEG-2.

Result contract: CC76XEKP5AI6AL7TX7O2XKXBD5A5HOUE4C6L36OVNO7XOKCSGWLDTCE6.

What it verifies — results

#	Check	Result
1	Go can upload contract wasm, hand-building the InvokeHostFunction XDR operation itself	✅ PASS
2	Go can create a fresh contract instance (CreateContract, address-based ContractIdPreimage + random salt) and recover the resulting contract address from the simulation's return value	✅ PASS
3	Go can drive the full lifecycle — initialize, fund, approve, release — including the simulate → attach footprint/fee/auth → sign → submit → poll cycle for each call	✅ PASS
4	Go-signed unauthorized calls are rejected the same way CLI-signed ones are	✅ PASS — see NEG-1
NEG-1	Organizer attempts release directly, signed and submitted by Go	✅ rejected on-chain (transaction included in the ledger but failed require_auth) — a stronger check than K01's CLI-level rejection, since this one actually reached the ledger's own auth enforcement, not just a local "missing signing key" short-circuit
NEG-2	Release the same escrow twice, via Go	✅ rejected — simulation itself failed with HostError: Error(Contract, #7) = Error::AlreadyReleased, same contract-level guarantee K01 confirmed

Findings folded into ADR-008

1. The Go SDK has no high-level "invoke a contract" helper — txnbuild.InvokeHostFunction exists, but building HostFunction/CreateContractArgs/ScVal values is manual, low-level XDR construction (github.com/stellar/go/xdr). Budget for this in E02 (Go EscrowClient) — it's not a thin wrapper over a convenience API, it's real integration work.
2. The simulate → sign → submit → poll pattern has to be hand-rolled too. clients/rpcclient (package name client, despite the directory being rpcclient) exposes SimulateTransaction/SendTransaction/GetTransaction/LoadAccount as raw JSON-RPC calls — there's no "preflight and assemble" helper. The pattern that worked: build an unsigned tx with empty Auth/Ext, simulate it, parse TransactionDataXDR into xdr.SorobanTransactionData and each Results[0].AuthXDR entry into xdr.SorobanAuthorizationEntry, set SorobanData.ResourceFee = MinResourceFee, rebuild the tx with those attached (txnbuild.NewTransaction auto-sums the classic + resource fee once Ext.SorobanData is set), sign, submit, poll GetTransaction for SUCCESS/FAILED.
3. Address arguments need to handle both G- and C- prefixed strkeys. Soroban's Address type is polymorphic over accounts and contracts; naively assuming every address argument is a G... account address broke on the token argument to initialize (a C... contract address) — see addressArg(), which tries AddressToAccountId first and falls back to a contract address.
4. A contract's CreateContract return value is recoverable from the simulation response (Results[0].ReturnValueXDR, an ScVal of type Address) — no need to hand-derive the CAP-46 contract-id-from-preimage hash to know what address was just created.

How to run

Requirements: Go 1.26+, the K01 wasm already built (cd ../k01-soroban-escrow && cargo build --target wasm32v1-none --release).

# from spikes/k02-go-soroban/
go mod tidy
go run .

No soroban-cli/stellar-cli needed for this spike — the whole point is that Go can do it independently.

Next step

K02 is done. Per server-build-plan.md's own sequencing, K03 (wallet compatibility check) and K04 (ADRs from K01–K03 findings) are next; S01 (monorepo scaffold) shouldn't start before K04 folds these findings in.